This invention relates generally to heterodyne receivers, and, more particularly to an improved Fabry-Perot diplexer for use within an optical heterodyne receiver.
The basic radio frequency heterodyne or superheterodyne receiver is well-known in the art. The receiver is primarily made up of the following elements: an RF amplifier (optional), a local oscillator, a mixer, an IF amplifier (optional) and a detector.
The RF amplifier (the first stage of the heterodyne receiver) generally provides an amplified signal for use within the receiver. The local oscillator generates an unmodulated RF signal whose frequency can be varied over a wide range to suit the range of carrier frequencies accepted by the receiver. The mixer accepts the amplified version of the carrier from the RF amplifier and the signal from the local oscillator. It combines or "mixes" these signals and produces a variety of signals in its output. Each signal bears the modulation of the RF carrier, but the frequency of one of them is equal to the difference between the RF carrier and the local oscillator frequency. This is the IF signal processed in the receiver. The IF signals derived from the mixer are too weak to be of much use and are therefore amplified in the IF amplifier. Assuming the IF signal has been amplified to a usable level, the next step in the evolution of the receiving chain is the incorporation therein of the detector. The detector extracts the intelligence from the modulated IF carrier for use at a desired time.
Unfortunately at shorter wavelengths or higher frequencies, that is, in the range of approximately 60-60,000 GHz (5mm to 5 .mu.m) it becomes difficult to construct an effective microwave receiver. High ohmic loss per unit length and close physical tolerance requirements produce practical limitations on the fabrication of waveguide devices which have dimensional tolerances that are necessarily fractions of a wavelength.
In order to overcome the shortcomings of the ratio frequency heterodyne receivers for operation at short wavelengths it has been demonstrated that the microwave network functions can be accomplished optically. The optical heterodyne receiver, which, throughout the specification, is one which includes infrared submillimeter and millimeter wavelengths, involves operations with diffraction limited beams of at least a few tens of wavelengths in diameter. Since the energy in such beams does not interact with any guiding structure, transmission losses are virtually non-existant.
One requirement, however, of the optical heterodyne receiver is that the input beams entering the mixer portion of the receiver be parallel. In order to provide for such a parallel beam relationship optical heterodyne receivers heretofore in use have relied upon a diplexer which generally is in the form of a folded Fabry-Perot resonator of the type described in "A Quasi-optical Radiometer", Digest of the Second International Conference on Submillimeter Waves and their Applications, San Juan, Puerto Rico, Dec. 6-10, 1976 by J. J. Gustincic.
Since the fields which incorporate the optical heterodyne receiver at short wavelengths (infrared, submillimeter and millimeter wavelength region), such as CO.sub.2 laser wavelengths as used in reconnaissance, communications, radar, imaging systems and pollution detection devices are an ever expanding area of usage, it is essential to provide an optical heterodyne receiver which is extremely economical to produce and highly effective in operation.
The optical heterodyne receivers in use today fail to meet the requirements set forth hereinabove since the diplexer used therein, such as the Fabry-Perot diplexer cited hereinabove relies upon high reflection (95% or higher, depending on the signal bandwidth) low transmission (5% or less) and extremely low loss (compared to transmission) mirrors in its construction. As a result thereof, such Fabry-Perot diplexers have large insertion losses because of the virtual impossibility of making extremely highly reflecting mirrors for use with short wavelengths which simultaneously have finite transmission and low absorption and scatter compared to the transmission. Since, heretofore, the solution to these problems have not been found, film coating for the infrared and meshes for the submillimeter and millimeter wavelength regions are conventionally used despite the existing problems.